After some fiddling, turns out the "Godray quality" being turned on drops my framerate a full 10 fps. According to Remedy's graphics settings guide, the Godrays are as follows:
So on one hand, it will only hurt my performance during the day time, when there's little action going on. On the other hand, there's virtually no difference as far as I can tell:
Pic1 Pic2
I took these a moment ago, the only difference being the Godrays settings being on or off (and a 10 fps drop when on). I don't actually know which is which.
[EDIT] Without the God Rays turned on, I'm getting around 40 fps with my 4890. High settings on most things, and 8x AA. Lowering AA only improved 1-2 fps.